The present invention relates to an aluminum nitride (AlN) powder suitable for use in the formation of green sheets which provide, after sintering, colored aluminum nitride sintered bodies useful as ceramic insulating substrates having good thermal conductivity and light shielding properties. The green sheets are particularly useful in the manufacture of multilayered ceramic substrates by the green sheet laminating technology. They are also useful as substrates for hybrid IC's. The present invention is also concerned with a process for preparing such an aluminum nitride powder.
Ceramic insulating substrates are usually produced by preparing a slurry containing a ceramic powder, an organic binder, and optionally one or more additives including a sintering additive, a dispersant, and a colorant, casting the slurry, e.g., by use of a doctor blade, to form a ceramic green sheet, and heating to sinter the green sheet to form a sintered ceramic body. Before or after sintering, an electrically conductive pattern is formed on the sheet by printing using a conductive paste.
Multilayered substrates are produced either by the green sheet laminating technology or by the green sheet printing technology. In the green sheet laminating technology, a plurality of ceramic green sheets prepared in the above manner and each having a printed conductive pattern formed thereon and, if necessary, through holes normally filled with a conductive material, are laminated. In the green sheet printing technology, a plurality of conductive layers and ceramic layers are formed alternately on a ceramic green sheet by printing with a conductive paste and a ceramic paste, respectively. In either method, the resulting multilayered structure is then sintered to produce a multilayered substrate having a multilayered interconnection of a conductor therein.
As the integration density of LSI's increases, it is important to quickly release the heat generated by the operation of LSI's. As a result, in place of conventional alumina substrates, aluminum nitride substrates have attracted increasing attention, since sintered aluminum nitride has a thermal conductivity much higher than sintered alumina and good high-temperature strength and electrical insulating properties.
There are two well-known methods for preparing aluminum nitride powder, which is used as a starting powder (sintering powder) in the production of sintered aluminum nitride. One is the direct nitriding method in which metallic aluminum powder is reacted with nitrogen or ammonia gas. The other is the alumina reduction method in which a mixture of alumina and carbon is reacted with nitrogen or ammonia gas to effect reduction of alumina and nitriding simultaneously.
The direct nitriding method gives a relatively coarse aluminum nitride powder, which must be finely divided by pulverization for a long period before use as a sintering powder. Such pulverization is inevitably accompanied by incorporation of impurities up to the order of a few percent and distortion of crystals. Therefore, it is very difficult in the direct nitriding method to obtain an aluminum nitride sintering powder of high purity having good physical properties including thermal conductivity and good stability in the presence of oxygen.
In contrast, the alumina reduction method provides a fine aluminum nitride powder which can be used as a sintering powder without further pulverization. Accordingly, the sintering powder is highly pure and has good physical properties. However, the aluminum nitride powder prepared by the alumina reduction method contains residual carbon, since carbon is used in excess. The residual carbon must be removed by burning (oxidative heat treatment in an oxidizing atmosphere such as air) before the aluminum nitride powder is used.
In order to accelerate the oxidative removal of carbon or decarbonization of an aluminum nitride powder prepared by the alumina reduction method, it is known to use an oxidation catalyst. Japanese Patent Application Kokai No. 62-191407 (1987) discloses that an oxidation catalyst selected from carbonates, nitrates, and oxides of alkali metals, alkaline earth metals, and rare earth metals is added to a carbon-containing aluminum nitride powder, and the mixture is heated in air at a temperature below 600.degree. C. to selectively remove carbon. It is proposed in Japanese Patent Application Kokai No. 2-120214 (1990) that a mixture of a carbon-containing aluminum nitride and at least one alkaline earth metal or rare earth metal compound be heated at a temperature of 650.degree.-900.degree. C. for 10 minutes to 6 hours in the presence of oxygen to obtain a decarbonized aluminum nitride powder.
In order to protect IC memories formed within substrates from light, particularly UV light and prevent them from showing unevenness of sintering and color, there is a need for colored aluminum nitride sintered bodies.
Japanese Patent Application Kokai No. 63-233079 (1988) describes a method for preparing such a colored aluminum nitride sintered body. In this method, calcium tungstate and/or calcium molybdate which serves as a color-developing agent is added to an aluminum nitride powder, and the resulting mixture is shaped and heated in a non-oxidizing atmosphere to yield a black-colored, light-shielding aluminum nitride sintered body.
It was confirmed that the colored aluminum nitride sintered body prepared by the above method has a decreased tendency toward unevenness of sintering and color and an increased yield of the product compared to a conventional, light-permeating sintered aluminum nitride. The presence of calcium tungstate and/or molybdate in the resulting colored sintered body is expected to have an additional effect of increasing the affinity of the sintered body for a conductive paste which usually contains tungsten or molybdenum powder as a conductive material, thereby facilitating the fixing of a conductive pattern on a substrate formed by the sintered body.
When the above-described method is employed in the production of a multilayered substrate by the green sheet laminating technology, there is a problem that delamination is often encountered during sintering of the laminated green sheets due to a failure of close contact between the green sheets and the printed conductive pattern.
Such delamination can be prevented by reducing the mixing period for preparing a slurry used to cast into green sheets, but this fails to form a uniform slurry, thereby interfering with uniform sintering and resulting in the formation of a sintered body having unevenness of color and sintering.